Refactor: cache duration

[katspaugh]

As suggested in #2890, since audio duration is polled on every tick, it would be beneficial to cache it until another audio file is loaded.

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Type	Occurrences	Fixable
Errors	0	0
Warnings	0	0
Ignored	0	N/A

• Result: ✅ success
• Annotations: 0 total

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